1. Field of the Invention
Vacuum distillation is historically the preferred method for concentrating or drying materials that may decompose at normal boiling temperatures. Obviously at lower than atmospheric pressures less kinetic energy and, therefore, lower boiling temperatures may be employed.
Evaporation procedures were greatly improved by the technique called "rotary evaporation." In this method, a solvent flask is rotated usually in a heated water bath, while the flask is under vacuum. The resulting thin film continually being developed as the flask rotates leads to rapid, low temperature evaporation with little or no "bumping" or necessity for a capillary leak or boiling stones.
Sublimation (also called lyophilization or freeze drying) is a process in which heat sensitive materials, such as proteins, hormones, antibiotics, vaccines and the like, may be safely dried for long term storage. Upon addition of water the freeze dried product reconstitutes to virtually the originally material. Viability of yeasts, molds, bacteria and viruses are often preserved by this process.
In sublimation, these effects are accomplished by first freezing the product solidly and then subjecting it to a high vacuum and controlled product heating. Under these conditions, the water goes directly from the solid ice state to water vapor--bypassing the liquid phase. The result of this gentle drying method is a dry product maintaining all or most of its original desirable characteristics. The dry product is usually stored under vacuum or inert gas, such as nitrogen. Usually no further refrigeration is required and the product may be stored for months or years at room temperature.
2. Description of the Prior Art
Vacuum flasks suitable for use in rotary evaporation or sublimation (freeze drying) are, for all practical purposes, identical to each other in their preferred sizes, shapes and materials of fabrication. Methods devised over the years at improving their everyday utility are, again, too similar to distinguish one from another. However, flasks for sublimation (freeze drying) are routinely subjected to much higher vacuums than rotary evaporator flasks and, therefore, both fabrication and suggested improvements must be at least to the standards of rotary evaporator flasks and, in fact, usually to much higher standards. For reasons of simplicity, therefore, the following discussion of the prior art will be limited to sublimation (freeze drying) flasks.
Orginally, freeze drying was accomplished by placing the material to be dried in a glass flask, then rotating the flask slowly in a dry ice and acetone (or other suitable solvent) bath until the contents were solidly frozen and finally connecting this flask, via rubber tubing or a rubber vacuum valve, to a steel manifold which had been previously evacuated to a high vacuum. The manifold in turn was connected in series between a dry ice and solvent refrigerated water vapor condenser and a high vacuum pump.
This manifold freeze drying method is still in wide use today except that mechanical (fluorocarbon) refrigeration for the water vapor condenser has largely replaced the use of day ice. The reasons for manifold freeze drying's continued use is the extreme simplicity of the method, particularly for biomedical research applications. The method does not require special training for successful use; samples are clearly observed during drying; the heat required for efficient sublimation is conveniently applied by ambient room temperature and moisture condensing on the flask surface; the end point, when the product is dry, can often be determined by simply touching the flask to check for cold spots not yet completely sublimated; typically many flasks, even containing different materials, may be processed at one time; flasks can be introduced and withdrawn during operation without disturbing other flasks; certain types of flasks can be easily disengaged to provide wide mouth access to the dried product.
Early attempts at freeze drying used pear-shaped glass containers with a standard taper ground glass neck which connected directly to a manifold. They worked fairly well, but the flask was fabricated in two pieces, each with a ground glass flange which could be secured together with silicone or other suitable vacuum grease for high vacuum operation. At the conclusion of a dehydration, the top and bottom sections were disengaged to provide wide mouth access to the dried product.
These classic flasks are still in use today since they provide the researcher with highly desirable total visability and rapid spark induction coil (Tesla coil) vacuum leak detection. However, two disadvantages limit their use. The major problem by far is the necessity for using vacuum grease. It is extremely difficult to prevent vacuum grease contamination of the product, which fact alone greatly limits their use for many applications, and vacuum grease itself is disagreeable to work with since it is difficult for technicians to remove it from their hands, clothing and virtually anything else it contacts.
The second problem with these flasks is that during freezing in a typical dry ice solvent bath, the flask must be manually or mechanically rotated in such a way as to prevent the material within the flask from contacting the flange juncture of the top and bottom and also to keep this greased junction itself away from harmful solvent action that could prevent a high vacuum seal from obtaining later. The resultant frozen plug is far from ideal for efficient heat transfer and sublimation rates, invariably leading to substantially longer drying times than would apply if it were permissible to freeze the sample in a horizontal position to achieve a layer of ice of uniform depth and maximum surface area for sublimation to occur.
In recent years numerous attempts have been made to overcome these difficulties, particularly with reference to eliminating vacuum grease. Silicone rubber gaskets and O-rings have been employed between rigid tops and bottoms, sometimes necessitating metal spring closures to prevent them from falling apart in handling, and invariably requiring the junction to be raised above the cold bath during freezing. And even if a clear elastomer was used, the spark induction coil (Tesla coil) leak detection would be difficult or impossible.
Rigid or semi-rigid caps have been used, presumably to avoid both the use of vacuum grease and to permit freezing the flask in a horizontal position. No clear explanation is given in the case of a totally rigid flask as to why the glass portion doesn't break when ice expands upon freezing. And in the case of a resilient cap, no explanation is given as to the reliability of such a seal upon ice expansion or how the heat transfer solvent is positively prevented from gaining access to the interior of the flask itself freezing, which would harm or destroy the product. In both cases, no provisions are made for Tesla coil leak inspection and their designs rule out modifications to make this possible.